I have found two of the tablets in Collasium. I have the one glowstone that the mermaid gave me and opened the door to the spinning room with the 4 exits. I used that to enter the cave crab prison and get the other tablet. There is another place to put a glowstone in that corridor, but i can't seem to find another one. Can anybody help me?

it's in a chest near an another mermaid, but i really don't remember where.

Thanks. Found 2 chests that were just hidden behind junk on the floor. One had what I was looking for, exactly where you said it was.